Conventionally, in order to detect the oxygen concentration of intake gas (the air) supplied from the outside to an internal combustion engine, the following technique is employed: a gas sensor is disposed in an intake gas passage of the internal combustion engine, and the detected oxygen concentration is used in operation control of the internal combustion engine, thereby improving accuracy in air-fuel-ratio control of the internal combustion engine (refer to, for example, Patent Document 1).
Also, in recent years, for the purpose of reducing nitrogen oxides (NOx) emitted from the internal combustion engine, a technique for reintroducing exhaust gas into an intake system (hereinafter, referred to as an exhaust gas recirculation system or an EGR system) is known (refer to, for example, Patent Document 2). In order to detect the oxygen concentration of intake recirculation gas, which is a mixture of exhaust gas from the EGR system and intake gas, disposing a gas sensor in an intake recirculation gas passage is known.
A known gas sensor to be installed in an intake passage, such as the intake gas passage or the intake recirculation gas passage, has a detection element in which electromotive force of different magnitude is generated or whose resistance varies, according to the concentration of, for example, NOx (nitrogen oxides) or oxygen (refer to, for example, Patent Document 3). While the gas sensor is exposed to a high-temperature gas to be measured, such as intake gas or intake recirculation gas, water contained in the gas adheres thereto (adhesion of water). Thus, the detection element is subjected to thermal shock and may be cracked. In order to cope with the problem, the gas sensor has a protector which covers the detection element, for protecting the detection element from adhesion of water. Further, although gas flowing through the intake passage contains much soot (carbon), the protector can prevent adhesion of soot to the detection element, thereby preventing deterioration in detection accuracy of the detection element.
The protector is provided in such a manner as to enclose a detection portion of the detection element for protecting the detection element from adhesion of water and soot. Meanwhile, in order to expose intake gas to the detection portion of the detection element, the protector has a gas inlet hole.